It is important to identify the signal transduction pathway involved in the regulation of fluid reabsorption by the ductuli efferentes of the testis because they reabsorb most of the fluid leaving the testis and are essential for male fertility. Microperfusion studies of the ducts in vivo showed that 0.1 or 1.0 mM dibutyryl (db)-cGMP in the perfusate had no effect on fluid reabsorption, but 0.1 mM db-cAMP significantly reduced fluid reabsorption, 0.25 mM abolished reabsorption, and 0.5–1.0 mM caused secretion. The inhibitory effect of db-cAMP was reversible. Although the presence of db-cAMP in the perfusate did not affect the concentration of Na in the collectate, the concentrations of K and Cl^– increased, indicating that their transport is at least partly regulated by cAMP. Including the phosphodiesterase inhibitor pentoxifylline in the perfusate decreased fluid reabsorption by the ducts in a dose-dependent manner, and it also increased the concentration of cAMP (5.5-fold) in collectate. Pentoxifylline also increased the production of cAMP (4-fold) by ducts incubated in vitro. It is concluded that cAMP, but probably not cGMP, is an intracellular messenger regulating fluid reabsorption in the efferent ducts.